Chewing insects are plant pests that bite off and consume solid pieces of leaf, stem, or root tissue, leaving behind holes, ragged edges, and sometimes completely stripped plants. They include some of the most common garden and crop pests: caterpillars, grasshoppers, beetles, and sawflies. Understanding how they feed, what their damage looks like, and how to control them can save you a lot of frustration in the garden or field.
How Chewing Mouthparts Work
Unlike aphids or whiteflies that pierce plants and suck out fluids, chewing insects have a set of jaw-like structures called mandibles that move side to side, not up and down. These mandibles have scissor-like edges for cutting and flat molar surfaces for grinding, essentially working like a tiny pair of pliers. Behind the mandibles sit a pair of smaller appendages that act like a fork and spoon, manipulating food into position. A tongue-like structure in the center mixes the chewed material with saliva before swallowing.
This mouthpart design is considered the most “primitive” insect feeding arrangement, meaning it evolved first. Many other feeding styles, like the piercing tube of a mosquito or the coiled straw of a butterfly, are modified versions of this same basic toolkit. Any insect with functional mandibles can damage plants by physically removing tissue, which is why chewing insects tend to cause the most visible destruction in gardens.
Common Types of Chewing Insects
Several major groups of insects fall into the chewing category, and each tends to attack plants in a slightly different way.
Caterpillars are the larval stage of moths and butterflies, and many are voracious defoliators that can strip a plant nearly bare. Tent caterpillars, cabbage loopers, tomato hornworms, and fall webworms are among the most familiar. You’ll often find silk webbing or small dark pellets of excrement near their feeding sites.
Beetles cause chewing damage in both their larval and adult stages. Flea beetles chew tiny shotgun-like holes in leaves, Japanese beetles skeletonize foliage by eating tissue between the veins, and powderpost beetles bore into wood. Some beetle larvae feed underground on roots before emerging as leaf-feeding adults.
Grasshoppers are strong fliers with large mandibles that tear irregular chunks from leaf margins. Eastern lubber grasshoppers are a particularly destructive species in the southeastern United States.
Sawflies look like caterpillars but are actually the larvae of wasp-like insects. They feed in groups and can defoliate shrubs and trees quickly.
Leafminers are a special case. These tiny larvae (often fly maggots about 1/8 inch long) burrow between the upper and lower surfaces of a leaf, creating winding trails or blister-like blotches. They’re technically chewing the tissue apart from the inside, but the damage pattern looks very different from external feeders. The citrus leafminer, actually a moth larva, is one well-known example.
Identifying the Damage
Chewing insect damage is usually easy to distinguish from other types of plant injury because tissue is physically missing. The specific pattern tells you a lot about what’s feeding.
- Irregular holes in leaves: The most common sign. Caterpillars, grasshoppers, and beetles all chew holes of varying sizes. Freshly damaged edges look green and moist; older damage turns brown and papery.
- Skeletonized leaves: When insects eat the soft tissue between leaf veins but leave the veins intact, the leaf looks like a lace skeleton. Japanese beetles and some caterpillar species feed this way.
- Complete defoliation: Heavy infestations of caterpillars or sawflies can remove every leaf from a branch or even an entire plant.
- Ragged leaf edges: Grasshoppers and larger caterpillars often start feeding from the margin inward, leaving torn, uneven borders.
- Winding trails inside leaves: Leafminers create pale, serpentine tunnels visible on the leaf surface.
- Stem tunneling and girdling: Borers chew into stems, petioles, and woody tissue. Twig girdlers cut a ring around small branches, causing them to break and fall.
Look for secondary clues, too. Silk threads near damaged leaves point to caterpillars. Small dark droppings (called frass) scattered on lower leaves or the ground below confirm that something is actively chewing overhead. Sawdust-like material at the base of a stem or trunk suggests a boring insect inside.
Prevention With Physical Barriers
The simplest way to stop chewing insects is to keep them off your plants entirely. Floating row covers, made from lightweight spunbond polyester or polypropylene fabric, act as a physical barrier draped directly over crops or supported on low hoops. They block insects, birds, and small mammals while still allowing light, air, and rain to pass through.
Timing matters. Place row covers over plants before pests become active in your area, typically in early spring when adults emerge and again in late July through early August for second-generation pests. Secure the edges every three feet with soil, rocks, or landscape staples so nothing crawls underneath. Inspect seedlings before transplanting to make sure you aren’t trapping eggs or larvae under the cover.
Crop rotation is another key prevention tool. Rotating crops by plant family each year means you aren’t laying row covers over soil where eggs may have been deposited the previous season. Regularly lifting covers to check for problems is important, since a few pests that sneak in can multiply unchecked in the protected environment.
Biological and Organic Controls
When chewing insects do get established, several organic options target them effectively without wiping out the beneficial insects you want to keep around.
Bt (Bacillus thuringiensis) is a naturally occurring soil bacterium that produces proteins toxic to specific insect groups. When a caterpillar eats leaf tissue coated with Bt, the proteins bind to receptors in its gut, causing it to stop feeding and die within a few days (up to ten days for larger species). Different strains target different pests: one strain kills caterpillars like cabbage loopers, hornworms, and bagworms; another targets beetle larvae; yet another works on fly larvae including mosquitoes and fungus gnats. Bt has no toxicity to mammals, fish, or other wildlife at recommended rates, and its residual life on foliage is less than ten days. It’s one of the most popular choices among home gardeners because it’s effective, affordable, and spares pollinators and predatory insects.
Neem-based products containing azadirachtin work against caterpillars, sawflies, flea beetles, and weevils through multiple pathways. They deter feeding, discourage egg-laying, and disrupt growth in immature insects. These products are especially effective against younger larval stages of chewing pests.
Spinosad is derived from another soil bacterium and controls a broader range of pests than most other biological products, including caterpillars, leafminers, beetles, and thrips. It attacks the nervous system of insects that ingest or contact it. Feeding stops within minutes, and death follows within about two days.
One important caution with all of these: even organic insecticides can kill beneficial predators like lacewing larvae that eat pest insects. Apply them selectively and only where damage is occurring rather than as a blanket spray.
When Treatment Is Worth the Cost
Not every chewing insect warrants intervention. In agriculture and even in home gardens, the concept of an economic threshold helps guide the decision. This is the pest density at which you should take action to prevent the population from growing large enough to cause real damage. Below that threshold, the cost of treatment (whether in money, time, or harm to beneficial insects) outweighs the damage the pests are doing.
For commercial crops, these thresholds are developed through field research that measures yield loss at different pest densities. For bean leaf beetles on soybean, for example, the action threshold is set at about 80% of the level that would cause economic losses. For home gardeners, the math is less formal, but the principle holds: a few holes in your basil leaves don’t justify spraying. A rapidly expanding caterpillar colony stripping your tomato plants does.
Consider the pest’s life stage and population trajectory. A handful of large, nearly mature caterpillars will soon pupate and stop feeding on their own. A wave of tiny, newly hatched larvae is a different story, since they have weeks of feeding ahead of them and are most vulnerable to Bt and other controls while still small. Matching your response to the actual threat saves effort and preserves the predators and parasitic wasps that provide free, ongoing pest control in a healthy garden.